1. Field of the Invention
The present invention relates to a dimming device and a display device capable of controlling the reflectance and transmittance of light.
2. Description of the Related Art
A phenomenon in which a metal thin film of yttrium (Y), lanthanum (La), or the like bonds to hydrogen to change into a hydride which can transmit visible light has been reported (specification of U.S. Pat. No. 5,635,729, and Huibert and six others, Nature, (U.K.), March 1996, vol. 380, pp. 231-234). Since this phenomenon is reversible, by adjusting the hydrogen pressure in the atmosphere, it becomes possible to cause the thin film to change between a metallic luster state and a transparent state.
By changing the optical characteristics of the above thin film so as to switch between a state exhibiting a metallic luster and a transparent state, it becomes possible to realize a dimming mirror which is capable of freely adjusting the reflectance/transmittance of light. If a dimming mirror is used as a windowpane of a building or an automobile, for example, it becomes possible to shield (reflect) or transmit sunlight as necessary.
Such a dimming mirror has, for example, a structure in which a palladium layer is formed on a yttrium thin film. The palladium has a function of preventing surface oxidation of the yttrium thin film, and a function of causing hydrogen molecules in the atmosphere to be efficiently changed into hydrogen atoms so as to be supplied to yttrium. When yttrium chemically bonds to hydrogen atoms, either YH2 or YH3 is formed. While YH2 is a metal, YH3 is a semiconductor and has a forbidden band width which is greater than the energy of visible light, and therefore is transparent.
Moreover, since changes of states between YH2YH3 occur rapidly (about several seconds) even at room temperature, it is possible to perform switching between a reflection (metallic luster) state and a transparent state depending on the amount of hydrogen content in the atmosphere.
As another material which is capable of such transitioning between metallic luster  transparent, a Mg2Ni thin film is disclosed in Japan Society of Applied Physics, 2001 Spring Meeting, 31-a-ZS-14, for example.
The above conventional technique has a problem described below.
Firstly, in order to change the optical state of a thin film, it is necessary to expose the thin film to an hydrogen atmosphere. Specifically, it is necessary to control the amount of hydrogen (hydrogen partial pressure) in an atmosphere gas which is in contact with the thin film. Therefore, it is difficult to realize a practical dimming device by using the aforementioned conventional constitution.
Secondly, in the dimming device according to the conventional technique, a thin film containing a material which is capable of transitioning between metallic luster  transparent (hereinafter referred to as “dimming material”), e.g., yttrium, is used as a dimming layer. Such a dimming device has the following problems.
The switching between the metallic luster and transparent states in the dimming layer (thin film) occurs as hydrogen ions permeate the dimming layer. However, even if the hydrogen pressure in the atmosphere changes, reactions with hydrogen ions may occur at the dimming layer surface, but permeation of hydrogen ions into the thin film would be difficult. Therefore, it is possible for some of the yttrium atoms or molecules containing yttrium to remain unreacted. As a result, it will be difficult to enlarge the difference in reflectance between the metallic luster state and the transparent state exhibited by the dimming layer. Moreover, in order to use the dimming device for a broader range of purposes, it would be necessary to reduce the time (switching speed) required by the yttrium thin film (which is the dimming layer) to transition between the metallic luster state and the transparent state.
In order to enlarge the difference in reflectance between the metallic luster state and the transparent state and reduce switching speed, it might be conceivable to improve the reaction efficiency of YH2 (or Y)  YH3 in the dimming layer, for example. However, in a conventional constitution which uses a thin film as a dimming layer, there are limits to improvement in the reaction efficiency.
Moreover, when applying a dimming device to a display apparatus, it would generally be advantageous if the dimming layer had a state in which light is diffuse-reflected. However, the dimming layer in the dimming device according to the conventional technique above mirror-reflects light in its metallic luster state, and therefore is difficult to be applied to a display apparatus.
Thirdly, in order to change the optical state of the thin film according to the conventional technique above, it is necessary to hydrogenate the material, contained in the thin film, which is capable of transitioning between metallic luster transparent, by exposing the thin film to a hydrogen atmosphere. Specifically, it is necessary to control the hydrogen amount (hydrogen partial pressure) in the atmosphere gas which is in contact with the thin film. Since such control of hydrogen amount is performed across the entire surface of the thin film, the optical state of the entire surface of the thin film changes.
Thus, conventional dimming devices are based on the premise that they are to be applied to the purpose of changing the optical state of the entire surface of the thin film such as a dimming mirror, and applications to a display apparatus have not been proposed. In order to realize an application to a display apparatus, it would be necessary to compartmentalize the thin film into a plurality of pixels, and control the optical state of each pixel. However, it would be difficult, and impractical, to control the hydrogen amount in the atmosphere gas with respect to each pixel.
The present invention has been made in view of the above circumstances, and it is intended to provide a dimming device which is capable of allowing the state of a thin film to transition between a metallic reflection state and a transmitting state, without relying on a control of the hydrogen amount (hydrogen partial pressure) in the atmosphere gas. Moreover, it is intended to provide a dimming device which comprises a dimming layer containing, in the form of particles, a material that is capable of transitioning between a metallic luster state and a transparent state, and which is therefore able to switch the optical characteristics of the dimming layer more rapidly and can be applied to a broader range of purposes. Furthermore, it is intended to provide a display device by utilizing the aforementioned dimming device.